As semiconductor trends continue toward decreased size and increased packaging density, every aspect of semiconductor fabrication processes is scrutinized in an attempt to maximize efficiency in semiconductor fabrication and throughput. Many factors contribute to fabrication of a semiconductor. For example, at least one photolithographic process can be used during fabrication of a semiconductor. This particular factor in fabrication processes is highly scrutinized by the semiconductor industry in order to improve packaging density and precision in semiconductor structure.
Lithography is a process in semiconductor fabrication that generally relates to transfer of patterns between media. More specifically, lithography refers to transfer of patterns onto a thin film that has been deposited onto a substrate. Transferred patterns then act as a blueprint for desired circuit components. Typically, various patterns are transferred to a photoresist (e.g., radiation-sensitive film), which overlies a thin film on a substrate during an imaging process described as “exposure” of the photoresist layer. During exposure, a photoresist is subjected to an illumination source (e.g. UV-light, electron beam, X-ray), which passes through a pattern template (e.g., a reticle) to print a desired pattern in the photoresist. Upon exposure to the illumination source, a radiation-sensitive quality of the photoresist permits a chemical transformation in exposed areas of the photoresist, which in turn alters the solubility of the photoresist in exposed areas relative to that of unexposed areas. When a particular solvent developer is applied, exposed areas of a photoresist are dissolved and removed, resulting in a three-dimensional pattern in the photoresist layer. Such pattern is at least a portion of the semiconductor device that contributes to final function and structure of the device, or wafer.
Efficient defect detection during lithography processes is an area of growing interest in the semiconductor industry. A defect in a patterned photoresist structure can be transferred to inferior layers of a semiconductor during a subsequent etch process in which the photoresist is employed. A defect or structural deformity can be the result of a defective reticle used to pattern the photoresist.
While wafer defect inspection systems have relatively high stage accuracy due to recent scrutiny of stage accuracy in wafer throughput, reticle defect inspection systems still lack an appreciable level of stage accuracy and are difficult to employ for analytical tests. Reticle defects can occur during fabrication of the reticle or during subsequent handling thereof, which can result in repeating wafer defects. A repeating wafer defect is one that occurs consistently on multiple wafers, as compared to a non-repeating defect, which affects a wafer individually. The danger associated with repeating defects is decreased throughput, and thus increased cost, in wafer production. Typically, a repeating defect indicates that a defect exists on the reticle, and not just on the wafer. With the increasing utilization of advanced reticle enhancement techniques, the effect of defects, even marginal defects, can be magnified when transferred to a wafer. If a photomask or reticle contains defects, even sub-micron in range, such defects can be transferred to a wafer during exposure. Defects on reticles can cause inaccurate patterns to form on the wafer. In addition, electrically active regions may not perform as desired, leading to an overall degradation of chip performance.
As the semiconductor industry continues to produce sub-micron and sub-half-micron design structures, the importance of reticle defect detection and improvements therein has become paramount. Reticle manufacture is governed largely by wafer critical dimension (CD), which is defined as the smallest allowable width of, or space between, lines of circuitry in a semiconductor device. As methods of wafer manufacture are improved, wafer CD is decreased, which in turn permits smaller reticle defects to slip past detection and be printed on a wafer. That is, reticle defects of a size that was once negligible are now capable of being printed on a wafer, resulting in reduced wafer yield and/or performance.
Soft defects on a reticle can arise, for example, from contact with a contaminated pellicle and/or from pellicle glue outgasing. Soft defects can also arise in many instances as a result of, for example, a sulfuric acid cleaning step during manufacturing of the reticle, and are typically comprised of a thin film of organic material. If a contaminant comprises, for example, sulfur, phosphorus, or amine(s), soft defect growth can be triggered upon exposure. Defects arising in this manner are often referred to as “adder” defects. Furthermore, soft defect growth is inversely proportional to wavelength value (e.g., the shorter the wavelength, the greater the rate of defect growth). As lithography processes become more refined, manufacturers employ shorter and shorter exposure wavelengths to reduce critical dimensions. This trend increases the risk of reticle soft defect growth, which in turn increases the probability that a defect will be printed on a wafer.
Conventional reticle inspection tools permit a user to inspect a reticle for soft defects and can provide some transmission data. For example, based on transmission loss associated with a particular inspection wavelength (e.g., 365 nm or 488 nm), a user can make a relatively informed decision regarding when to send a reticle back to the mask shop for cleaning and pellicle replacement. However, this is a destructive method whereby the pellicle must be removed for cleaning and then replaced by a new pellicle. For example, in a case where there is no sulfur or phosphorus signature in the reticle defect, a user can waste valuable resources attempting to correct a defect that is merely cosmetic. Furthermore, a repeating defect can be created via sulfuric acid cleaning processes performed on a reticle that otherwise initially had no sulfur signature, thereby adversely affecting throughput. Thus, there exists a need in the art for systems and methods that can improve soft defect detection in real time.